1. Field of the Invention
The present invention relates to a seam welding apparatus and a seam welding method for welding a workpiece while the workpiece is fed between a pair of electrode rolls.
2. Description of the Related Art
Fuel tanks for use on motor vehicles or the like, for example, are manufactured by superposing outer peripheral ends of two members that have been pressed into a given shape, and then welding the superposed outer peripheral ends. The welded outer peripheral ends may have a two-dimensional or three-dimensional curved shape.
A seam welding method according to the related art employs an automatic copying apparatus, as disclosed in Japanese Laid-Open Patent Publication No. 04-294871. The automatic copying apparatus comprises a rotary table supported rotatably in horizontal directions, a workpiece rest supported on the rotary table for securing a workpiece thereto, a web-like copying cam mounted on the lower surface of the rotary table, which approximates a welding reference line of a flange section (including straight flanges and corner flanges) of a workpiece secured to the workpiece rest, and a pair of guide rollers for gripping the copying cam on both sides thereof at a position below upper and lower rotary electrodes on an electrode pressing central line. When the rotary table is rotated, a pressing point of the upper and lower rotary electrodes moves relatively along the welding reference line of the flange section.
According to the seam welding method disclosed in Japanese Laid-Open Patent Publication No. 04-294871, when welding conditions are changed, e.g., when the welding rate (the rotational speed of the rotary table) is increased, a track that is followed by the actual pressing point of the upper and lower rotary electrodes, i.e., the welded track, tends to vary. As a result, the upper and lower rotary electrodes may become displaced (derailed) from the workpiece, or may bite into the main body of the workpiece, which greatly impairs product quality.
One solution to the above problem would be to bring the welded track into conformity with the welding reference line. However, this solution is time-consuming and requires a large number of workpieces to be consumed on a trial basis, because it is necessary to repeat a process of establishing a welding reference line on the basis of CAD data, a process of fabricating a copying cam based on the established welding reference line, and a process of carrying out seam welding under various welding conditions.
The above difficulty has also been experienced by a seam welding process for seam-welding a workpiece while the workpiece is moved along an established path by a six-axis robot, as well as by a seam welding process using a copying cam.